Why is the Fields Medal awarded only to mathematicians under forty? Because everyone knows that people are at their most energetic and mentally active before forty. After forty, even if one's health is still good, mental prowess begins to decline significantly.
So, if there are no significant achievements before forty, it becomes very difficult after forty.
This is even more the case after sixty, as one gradually steps into old age. What remains after time has settled is only experience. At this stage of doing research, not only does mental power lag behind, but energy is lacking as well. The hope might only rest on disciples who can inherit one's legacy.
An outstanding student naturally excites admiration from the greats.
Zhang Shuwen certainly understands this mentality because he feels the same way, which is why he chose to return and explain things face to face.